In most vehicles, braking is performed hydraulically, with a master cylinder developing suitably proportioned brake pressures for the front and rear wheels of the vehicle in response to driver brake pedal pressure. Alternatively, it has been proposed in U.S. Pat. No. 5,015,040, issued on May 14, 1991, assigned to the assignee of the present invention, and incorporated herein by reference, to provide brake effort proportioning with an electronic control system that brakes the wheels of one axle to achieve wheel speeds that match the corresponding wheels of the other axle. Some vehicles additionally include an active brake control (ABC) which overrides the normal braking under conditions of lateral instability to produce differential braking as between the left and right wheels of the vehicle. The differential braking induces a desired yaw moment that may be determined either open-loop (that is, in response to driver inputs, such as steering wheel angle) or closed-loop (that is, in response to a deviation between desired and measured parameters, such as yaw rate or side slip velocity). In either case, the desired yaw moment is typically considered as a desired speed differential as between the left and right wheels, and the differential braking is controlled to achieve the desired speed differential. In general, the control is carried out by isolating the brakes of the driven axle (front or rear) from the master cylinder, and then individually adjusting the those brake pressures so that one of the driven wheels is allowed to free-wheel while the other driven wheel is braked as required to achieve the desired speed differential.
While ABC can greatly enhance the handling of the vehicle, it also disturbs the normal front-to-rear proportioning of brake pressure carried out by the master cylinder. This phenomenon is especially noticeable in front-wheel-drive vehicles since the ABC reduces the overall braking effort at the front wheels, where the braking effort is usually concentrated. Typically, the driver reacts by increasing the brake pedal pressure, and the increased braking effort is provided by the un-driven wheel brakes. If the braking produces substantial wheel slip, an anti-lock brake control overrides the ABC control, at least to the extent required to stop the slipping.